The known capsules, used in machines for making extraction or infusion beverages, comprise in their simplest form:                a rigid, cup-shaped outer container comprising a perforatable or perforated bottom and an upper aperture provided with a rim (and usually, but not necessarily, having the shape of a truncated cone);        a dose of product for extraction or infusion beverages contained in the outer container;        a length of sheet obtained from a web for sealing (hermetically) the aperture of the rigid container and designed (usually but not necessarily) to be perforated by a nozzle which supplies liquid under pressure.        
Usually, but not necessarily, the sealing sheet is obtained from a web of flexible material.
In some cases, the capsules may comprise one or more rigid or flexible filtering elements.
For example, a first filter (if present) may be located on the bottom of the rigid container.
A second filter (if present) may be interposed between the piece of sealing sheet and the product dose.
The dose of product may be in direct contact with the rigid, cup-shaped outer container, or with a filtering element.
The capsule made up in this way is received and used in specific slots in machines for making beverages.
In the technical sector in question, the need is particularly felt for filling in a simple and effective way the rigid, cup-shaped containers or the filtering elements whilst at the same time maintaining a high productivity.
It should be noted that, in this regard, there are prior art packaging machines having a filling unit which allows the simultaneous filling of several parallel rows of rigid, cup-shaped containers, which are advancing.
In this case, each row of rigid, cup-shaped containers is associated with a dedicated filling device, generally equipped with a screw feeder to allow the descent of the product inside the container.
This type of unit is therefore obviously quite expensive and complex, since it comprises a plurality of devices and drives (one for each screw device) which are independent from each other and which must necessarily be coordinated.
Moreover, the overall reliability of the machine resulting from this configuration/arrangement of elements is necessarily limited because the rate of faults is inevitably linked with the number of devices and drives present.
Moreover, the screw feeder devices may have drawbacks due to clogging, soiling and poor dosing accuracy. More in detail, the end part of the screw feeder is not normally able to retain the product, which therefore falls and soils the machine.
A strongly felt need by operators in this sector is that of having a unit and a method for filling containing elements (rigid, cup-shaped containers) of single-use capsules for extraction or infusion beverages which are particularly simple, reliable and inexpensive and at the same time maintain a high overall productivity.